This invention relates generally to light sources having relatively large apertures and, more particularly, to semiconductor laser light sources having a large aperture and relatively high output power. Some applications of laser light sources, such as communications in space or high-density optical recording, require a relatively powerful light source from a wide aperture, but with full phase coherence across the aperture, and with optical-mode stability.
Briefly, by way of general background, a semiconductor diode laser includes a diode structure, having an n-type layer, a p-type layer, and an undoped active layer sandwiched between them. When this diode structure is electrically forward-biased in normal operation, electrons and holes from the n-type and p-type layers recombine in the active layer, and light is emitted. Cladding layers of appropriate index of refraction confine the light "transversely," in a direction perpendicular to the layers, and various techniques may be used to confine the light "laterally," i.e. in a direction perpendicular to a desired "longitudinal" direction of light output. Reflective facets are located at opposite ends of the structure, to reflect light back and forth in the longitudinal direction.
Single-element diode lasers are limited in power to outputs of the order of 30 milliwatts (mW), but arrays of diode lasers can be designed to provide output powers of hundreds of milliwatts. Developments in this area have in recent years been concerned with refinements in laser arrays, to produce beams of high output power and with desirable characteristics. For most high-power semiconductor laser applications there is a requirement for a diffraction-limited beam, i.e. one whose spatial spread is limited only by the diffraction of light, to a value roughly proportional to the wavelength of the emitted light divided by the width of the emitting source. Because of the requirement for a stable diffraction-limited beam, much research in the area has been directed to index-guided laser arrays, in which dielectric waveguides aligned in the longitudinal direction confine light in the lateral direction.
Most semiconductor structures employed for lateral index guiding in laser arrays are known as positive-index guides, i.e. the refractive index is highest in regions aligned with the laser elements and falls to a lower value in regions between the elements, thereby effectively trapping light within the laser elements. Another type of index guiding is referred to as negative-index guiding, or antiguiding, wherein the refractive index is lowest in the regions aligned with the laser elements and rises to a higher value between elements. Some of the light encountering the higher refractive index material will leak out of the lasing element regions; hence the term "leaky-mode" laser array is sometimes applied. The use of antiguides in laser arrays is disclosed in various patents and publications, for example U.S. Pat. No. 4,860,298 to Dan Botez et al., entitled "Phase-Locked Array of Semiconductor Lasers Using Closely Spaced Antiguides."
An important concept concerning laser arrays is that they may oscillate in one or more multiple possible configurations, known as array modes. In what is usually considered to the most desirable array mode, all of the emitters oscillate in phase. This is known as the fundamental or 0.degree.-phase-shift array mode, and it produces a far-field pattern in which most of the energy is concentrated in a single lobe, the width of which is limited, ideally, only by the diffraction of light. When adjacent laser emitters are 180.degree. out of phase, the array operates in the 180.degree.-phaseshift array mode, or the out-of-phase array mode, and produces two relatively widely spaced lobes in the farfield distribution pattern. Multiple additional modes exist between these two extremes, depending on the phase alignment of the separate emitters, and in general there are N possible array modes for an N-element array. Many laser arrays operate in two or three array modes simultaneously and produce one or more beams that are typically two or three times wider than the diffraction limit.
It is usually a goal in the design of laser arrays to produce a beam that is stable in its mode of operation, even at high powers, since it is difficult to achieve a diffraction-limited beam if the array mode is subject to change. Such mode instability can result from changes in refractive index, which may be thermally or optically induced at higher powers. U.S. Pat. No. 4,985,897 to Botez et al., entitled "Semiconductor Laser Array Having High Power and High Beam Quality," discloses a laser array of antiguides that overcomes this difficulty by operating in a resonance condition, such that there is very strong coupling between the waveguides, and improved device coherence. The structure disclosed in U.S. Pat. No. 4,985,897 also affords improved control over, and stability of the mode of operation, and provides for various techniques for suppressing unwanted modes.
Some applications of laser light sources require relatively large output apertures, larger than those of laser arrays disclosed in the patents discussed above. Prior attempts to achieve high power outputs from large apertures have not been completely successful because of the difficulties of maintaining device coherence and optical-mode stability across the large aperture. Furthermore, phase coherence in arrays does not insure single-frequency operation. Some applications, such as coherent optical space communications, require a diffraction-limited beam as well as single-frequency operation. Two approaches have been commonly employed, and both make use of a highly monochromatic laser, as a master oscillator providing the primary source of radiation. In one approach, a master oscillator is coupled to a semiconductor power amplifier having a lateral dimension that flares out to provide the desired wide aperture. This combination is known as a master oscillator with power amplifier (MOPA). In the other common approach, a master oscillator is coupled to a large-aperture semiconductor laser, which then oscillates at the frequency of the master oscillator. This combination is known as an injection-locked oscillator (ILO).
Basically, the principal difference between these two configurations is that the power amplifier is a one-pass device. Light entering one end of the amplifier is amplified as it passes to the other end, which has a wider aperture. If the amplifier is simply a large pn junction region of diverging width, and no waveguide structure, phase coherence of the device is not usually a problem but there is little or no stability in the optical mode of the output. The power amplifier may alternatively be constructed as a diverging tree structure of waveguides. Output from the master oscillator enters the amplifier through single waveguide and encounters a Y junction, which bifurcates the waveguide into two sections. These connect to additional Y junctions, from which additional waveguide sections connect to still more Y junctions, and so forth across the amplifier, with a large number of parallel waveguide sections producing output through a large aperture. In this version of the MOPA, mode stability is obtained but phase coherence is extremely difficult to control, because of the possible differences in the path lengths of the waveguide tree structure.
The injection-locked oscillator scheme works well only up to levels of 1.5 to 2.0 times threshold current in the laser, because above these levels a phenomenon known as gain spatial hole burning tends to spoil the mode purity of the laser. Again there is a need to control the optical mode in the plane of the junction (in the lateral direction) while maintaining full coherence across the aperture.
It will be appreciated from the foregoing that there is a need for a light source that overcomes the disadvantages discussed above, and which may be configured in various ways, such as a power amplifier or an injection-locked oscillator. The present invention satisfies this need.